Great Lakes Dairy Sheep Symposium - the Department of Animal ...

EPA and DHA in sheep milk is similar to that of cows’ milk, with fish oil, and to a greater
extent, rumen-protected fish oil allowing for an increase in the omega-3 content of sheep milk
(Chikunya et al., 2002; Kitessa et al., 2003). There has also been some success in the use of
marine algae to alter EPA and particularly DHA in growing lambs (Cooper et al., 2004).
The intake of cis-9, trans-11 CLA in humans is of interest because of the potential health
benefits it may confer. In particular, the anticarcinogenic activity of cis-9, trans-11 CLA has
been clearly established with in vitro cell culture systems and in vivo animal models for a wide
range of cancer types, with the antiatherogenic properties also now being established (Bauman et
al., 2005a). The major dietary sources of cis-9, trans-11 CLA are foods derived from ruminants,
with about 70% and 25% coming from dairy products and red meat, respectively, with cis-9,
trans-11 CLA representing >90% of total CLA found in ruminant products (Parodi, 2003). The
presence of cis-9, trans-11 CLA in ruminant milk and meat is related to rumen fermentation and
originates mainly from endogenous synthesis in the mammary gland; only a minor portion comes
from production in the rumen. The substrate used to form cis-9, trans-11 CLA is vaccenic acid
(trans-11 18:1) produced as an intermediate during rumen biohydrogenation, and the enzyme
that catalyzes the reaction is Δ 9 -desaturase (Figure 4). Due to the precursor:product relationship
between vaccenic acid and cis-9, trans-11 CLA, a close linear relationship has been reported for
the milk fat content of these fatty acids in dairy cows. This same relationship is shown in Figure
5 for sheep milk fat from our study reported above and a similar relationship was also observed
by Nudda et al. (2005).
Figure 4. Pathways for the biosynthesis of cis-9, trans-11 conjugated linoleic acid (CLA) in
ruminants. Adapted from Bauman and Lock (2005).
Linolenic acid
cis-9, cis-12, cis-15 18:3
cis-9, trans-11, cis-15 18:3
trans-11, cis-15 18:2
Rumen
Linoleic acid
cis-9, cis-12 18:2
cis-9, trans-11 CLA
trans-11 18:1
18:0
Numerous studies have shown that diet is the most significant factor affecting the cis-9,
trans-11 CLA content of milk fat, and its concentration can be increased several-fold by dietary
means (Lock and Bauman, 2004). The key to increasing milk cis-9, trans-11 CLA is to increase
rumen vaccenic acid output, allowing for increased endogenous synthesis in the mammary gland.
One strategy is to increase the dietary intake of 18-carbon PUFAs by addition of seeds or plant
oils high in linoleic and/or linolenic acids (e.g. soybeans or sunflowers) which results in an
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Tissues
cis-9, trans-11 CLA
D 9 -desaturase
trans-11 18:1